1. The Field of the Invention
The present invention relates generally to mounting systems for securing one or more components mounted to a rotatable shaft. More particularly, embodiments of the present invention relate to target anode mounting systems and devices that include various features which serve to reliably and effectively establish and maintain the position of the target anode in a variety of operating conditions, and that serve to redistribute at least some of the stress imposed on the target anode and other components as a result of operating conditions.
2. Related Technology
X-ray producing devices are valuable tools that are used in a wide variety of industrial, medical, and other applications. For example, such equipment is commonly used in areas such as diagnostic and therapeutic radiology, semiconductor manufacture and fabrication, and materials analysis and testing. While they are used in various different applications, the different x-ray devices share the same underlying operational principles. In general, x-rays, or x-ray radiation, are produced when electrons are produced, accelerated, and then impinged upon a material of a particular composition.
Typically, these processes are carried out within a vacuum enclosure. Disposed within the vacuum enclosure is an electron generator, or cathode, and a target anode, which is spaced apart from the cathode. In operation, electrical power is applied to a filament portion of the cathode, which causes a stream of electrons to be emitted by the process of thermionic emission. A high voltage potential applied across the anode and the cathode causes the electrons emitted from the cathode to rapidly accelerate towards a target surface, or focal track, positioned on the target anode.
The accelerating electrons in the stream strike the target surface, typically a refractory metal having a high atomic number, at a high velocity and a portion of the kinetic energy of the striking electron stream is converted to electromagnetic waves of very high frequency, or x-rays. The resulting x-rays emanate from the target surface, and are then collimated through a window formed in the x-ray tube for penetration into an object, such as the body of a patient. As is well known, the x-rays can be used for therapeutic treatment, or for x-ray medical diagnostic examination or material analysis procedures.
In addition to stimulating the production of x-rays, the kinetic energy of the striking electron stream also causes a significant amount of heat to be produced in the target anode. As a result, the target anode typically experiences extremely high operating temperatures, as high as 2400° C. during normal operations. However, the anode is not the only element of the x-ray tube subjected to such operating temperatures.
In particular, components such as the rotor stem, and the nut which secures the anode on the rotor stem, are also exposed to these high temperatures as a result of their proximity to, and substantial contact with, the anode. Additional heat is also generated by those electrons that strike the target surface but do not generate x-rays, and instead simply rebound from the surface and then impact another “non-target” surfaces within the x-ray tube evacuated enclosure. These are often referred to as “secondary” electrons. These secondary electrons retain a large percentage of their kinetic energy after rebounding, and when they impact these other non-target surfaces, a significant amount of heat is generated.
The heat produced by secondary electrons, in conjunction with the high temperatures present at the anode, often reaches levels high enough to damage portions of the x-ray tube structure. Thus, the joints and connection points between x-ray tube structures may be weakened when repeatedly subjected to such thermal stresses.
The destructive structural effects often observed in x-ray devices are not solely a function of high temperatures however, but may also be exacerbated by the relative rate of change of the temperature, or thermal stress cycling, that occurs over time. For example, the temperature in the anode region may, in some cases, increase from about 20° C. to about 1250° C. in a matter of minutes. The relatively rapid rate at which such temperature changes take place, coupled with the magnitude of the temperature change, imposes significant thermal stress and strain in the x-ray device components, and can ultimately lead to the permanent deformation and/or failure of such components.
In addition to being exposed to extreme thermal stresses, many components of the x-ray device, such as the anode and the target surface of the anode, as well as the mounting system and devices used to secure the anode to the rotor stem, are also subjected to high levels of mechanical stress induced by high speed rotation of the anode and rotor stem. For example, in many rotating anode type x-ray devices, the anode, the rotor stem and the nut used to attach the anode to the rotor stem, are subjected to high stress “boost and brake” cycles. In a typical boost and brake cycle, the anode may be accelerated from zero to ten thousand (10,000) revolutions per minute (RPM) in less than ten seconds. This high rate of acceleration imposes significant mechanical stresses on the anode, the rotor stem and the nut. Thus, the components which are used to secure the anode in position are exposed not only to extreme thermal stresses, but are simultaneously exposed to significant stresses imposed by the mechanical operations of the x-ray device.
The rotor stem, the nut, and the target surface of the anode are particularly vulnerable to such thermal and mechanical stresses, at least in part as a consequence of their chemical compositions and physical configurations. In particular, while the aforementioned components are all typically comprised substantially of low thermal expansion metals, it is often the case that these components heat up at different rates within the typical x-ray device operating temperature range of about 25° C. to about 1300° C. That is, while such components may generally expand to about the same extent, the speed with which they achieve such expansion typically varies from one component to another.
One consequence of the mismatch in thermal expansion rates is that one or more of the components may become permanently deformed over a period of time. As a result of such deformation, the components no longer fit tightly together and may vibrate during operation of the x-ray device. Such vibration is problematic at least because it increases the noise level associated with operation of the x-ray device, and because it contributes to motion of the focal spot on the target surface of the anode. Generally, motion of the focal spot is undesirable because it tends to compromise the quality of the images that can be obtained with the x-ray device.
The vibration problems resulting from the permanent deformation of one or more of the components used to secure the anode in position are further exacerbated by characteristics commonly encountered in known designs. For example, in many rotating component applications and environments, such as rotating anode type x-ray devices, a gap is defined between the outside diameter of the rotor stem and the opening in the anode through which the rotor stem passes.
Generally, the purpose of such a gap is to permit manipulation of anode orientation prior to operation of the x-ray device. In particular, the gap allows the assembler to attempt to minimize anode run-out with respect to the rotor stem by shifting the position of the anode slightly. However, while such a gap is useful in that it permits initial positioning of the anode with respect to the rotor stem, the gap also allows undesirable lateral movement, or radial runout, of the anode when the anode is subjected to mechanical and thermal stresses. Failure to compensate for such radial runout by limiting or preventing the movement of the target anode often results in problems with the operation of the device. For example, such radial runout frequently causes vibration and noise during operation of the x-ray device.
Another problem stemming from the extreme operating temperatures to which the target surface of the anode, the nut, and the rotor stem are exposed concerns the forces, including axial forces, that result from such extreme temperatures. As suggested by their name, “axial” forces generally refer to those forces exerted in a direction substantially parallel to the longitudinal axis of the rotor stem.
It was noted above that heating of the rotor stem, target surface, and nut during operation of the x-ray device causes those components to expand. Much of this thermal expansion occurs in a generally axial direction. However, because typical mounting systems are unable to accommodate or otherwise compensate for this thermal expansion, the thermal expansion of the individual components collectively acts to compress the rotor stem, target surface and nut, and thereby impose large compressive forces on those components. In some cases, the forces thus exerted are of a magnitude sufficient to permanently deform one or more of the components. As noted above, such deformation may induce, among other things, vibration and undesirable focal spot motion.
In view of the foregoing problems, and others, a need exists for a component mounting system that facilitates reliable and maintainable positioning of the mounted component on a rotary shaft, and that is able to compensate for, or otherwise mitigate, stresses imposed on the mounting system and other components as a result of operational conditions.